Urea, or diesel exhaust fluid, is a substance the car injects into the exhaust. It reacts with and neutralizes nitrogen emissions. It's required in the US and Canada because of emissions regulations, but is not present in most cases elsewhere in the world. Although pretty soon it'll start appearing in Europe as well.

To the OP, that warning means you need to top off the urea fluid or the car will not restart after 900 miles.

I haven't had to do this yet, but I did ask the dealer when I bought it how to DIY the refill. My notes at this point are skimpy, but what I noted was that I should push in the little panel bear the left rear wheel and fill the top reservoir. There is a top and bottom reservoir opening. What I am wishing to confirm is whether you fill both reservoirs in some sequence, or just the top, or just the bottom?

It's actually the bottom reservoir you must fill; it is connected to the primary, or "active" tank, from which the DEF injection system gets its working supply.

The top filler is for the secondary (passive) tank, which you can try and fill (but it's a bit messier since the tank is above the level of the filler tube, you will always get some DEF burping back at you when you fill the passive tank, no two ways about it).

As an aside, most larger DEF jugs (2 gal) are starting to come with the narrow insertion nozzle that will fit into the fillers.

OP: Typically the DEF should last till the next oil change when the dealer will refill at the same time.
You can ask your dealer to top off if you're not due for an oil change.
Check your iDrive which will indicate when your next oil change is due.
If it's soon you can schedule both service with your dealer.

It comes to about $11 after the 20% discount or w/e coupon they have going on.
It comes with a filler tube, but its too big to fit into the DEF refill hole on the 335d so a small funnel will be needed. As mentioned there are 2 holes, one for the main tank (lower hole) and one for the backup tank. I didn't have to use all 2.5 gallons to fill both tanks (I probably used about 2 gallons). After refilling, the message goes away on its own. There's no need to reset anything.